Breastfeeding produces not only healthier babies but also brighter children, according to a study. As little as four weeks of breastfeeding for a new-born infant has a "positive and significant effect" on brain development, it finds, right up to secondary school and beyond.

Children who had been breastfed consistently outperformed their formula-fed peers at ages five, seven, 11 and 14 in tests of reading, writing and mathematics, researchers from the University of Oxford and the Institute for Social and Economic Research at the University of Essex found.

The NHS already recommends that babies should be breastfed rather than given formula for their first six months, but Britain has one of the world's lowest rates of breastfeeding. Only 35% of UK infants are exclusively breastfed at one week old, 21% at six weeks, 7% at four months and just 3% at six months.

One of the paper's co-authors, Maria Iacovou, a social scientist at the Institute for Social and Economic Research, said that while the health benefits of breast milk were widely known and understood, it had been less clear to what extent there were benefits for cognitive development.

"The issue was that while it looked as though breast feeding did have an impact on cognitive development, no one knew if that was just because the type of mother more likely to breastfeed in the first place was more likely to nurture brighter children, or whether there was a true causal link," she said.

The ISER study – a working paper that will be peer-reviewed at a later date – compared breastmilk-fed children with formula-fed "twins", children who were equivalent in all other observable respects. "We did find there is a link [between breast milk and cognitive development]," said Iacovou. "Breast milk has well-known health benefits and now we can say there are clear benefits for children's brains as well."

Despite the findings, she said, she still supported mothers who decided for whatever reason that breastfeeding wasn't for them. "It really, really wasn't my intention to make any mother feel guilty," she said. "All this talk about bringing up children would sometimes seem to have us think that the child is the only thing that matters. Mothers are people too and have feelings and if you don't want to breastfeed your baby, well, luckily in this country you are not going to cause it harm. They would just do a little bit less well.

"I think we have a lot of challenges to change our culture and attitudes to breastfeeding but it's likely to happen through a gradual process of normalisation. Don't pressurise women who don't want to breastfeed but we should start focusing more on those women who do want to and try to help them pull it off and make it more normal for everyone."

Janet Fyle, professional policy adviser at the Royal College of Midwives, welcomed the research and said that, while it was interesting and further proof of the benefits of breastfeeding, there were several reasons behind the popularity of bottle-feeding infants in the UK.

The UK had a long way to go in normalising breastfeeding in the way other countries had done, she said. "In Sweden most mothers breastfeed because they are not forced to go straight back to work. In this country we are cutting benefits to single parent families and poorer people and mothers have to get back to work and earn a living, whether it's in Sainsbury's or the City – it is what the government wants them to do.

"So we are restricting them from breastfeeding there, while in society breastfeeding mothers are still getting chucked out of cafes and out of libraries. It's a problem.

"Often seeing the mothers who really need to be breastfeeding, really needing their children to be getting those cognitive and health benefits, are the ones least likely to. Then we look at young women today and the messages they are getting from magazines, from television, from certain parts of the media, about what breasts are actually for – there is another area we need to tackle."

She added that it was vital no one should be made to feel guilty. "Cleverer children is one thing that people have never pushed before – it's very easy to make mothers or families feel guilty about the choices they make and that is not in anyone's interest. We need to engage with mothers but also we need to take a softly-softly approach."

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